android应用之SQLite数据库

android系统内嵌了SQLite数据库，优点是处理速度快，占用内存少

在android应用中，围绕着数据库的操作主要是通过SQLiteDatabase来完成的，该类封装了以下方法：

方法	用途
execSQL(sql)	执行sql脚本，通常用来创建或管理数据库表格
rawQuery(sql,selectionArgs)	执行select查询操作(sql语句的方式)
query(..)	执行select查询操作(api的方式)
insert(table,nullColumnHack,values)	向指定table中新增记录
update(table,values,whereClause,whereArgs)	更新指定table中的指定记录
delete(table,whereClause,whereArgs)	删除指定table中的指定记录

通过这些方法便可完成对数据库的增删改查操作，同时SQLiteDatabase也引入了对事物的支持
beginTransaction()：开启事物
endTransaction()：提交或回滚事物
相对于web应用来讲，移动客户端的本质区别在于系统的访问是单用户的，没有并发操作。因此，在数据库不存在并发写操作的前提下，事物的处理是可以不去考虑的。

SQLiteDatabase是通过SQLiteOpenHelper来获取的，该类是一个抽象类，开发基于数据库存储的android应用时，需要继承该类，构建出自己的SQLiteOpenHelper实例并覆盖其超类所声明的抽象方法：
onCreate(SQLiteDatabase db)：用于创建数据库表格
onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion)：用于更新数据库表结构
有了SQLiteOpenHelper之后，便可通过其getWritableDatabase()方法获取SQLiteDatabase对象，用以完成对数据库的CURD操作。

public class WordSQLiteHelper extends SQLiteOpenHelper{
	public static final String TABLE_WORDS = "words";
	public static final String COLUMN_ID = "_id";
	public static final String COLUMN_ENGLISH = "english";
	public static final String COLUMN_CHINESE = "chinese";
	public static final String COLUMN_PHRASE = "phrase";
	public static final String COLUMN_TRANSLATION = "translation";
	public static final String COLUMN_LEVEL = "level";
	private static final String DATABASE_NAME = "words.db";
	private static final int DATABASE_VERSION = 1;

	public WordSQLiteHelper(Context context){
		super(context, DATABASE_NAME, null, DATABASE_VERSION);
	}
	@Override
	public void onCreate(SQLiteDatabase db) {
		StringBuilder sql=new StringBuilder("create table ").append(TABLE_WORDS).append("(")
				.append(COLUMN_ID).append(" integer primary key autoincrement,")
				.append(COLUMN_ENGLISH).append(" text not null,")
				.append(COLUMN_CHINESE).append(" text not null,")
				.append(COLUMN_PHRASE).append(" text not null,")
				.append(COLUMN_LEVEL).append(" real not null,")
				.append(COLUMN_TRANSLATION).append(" text not null);");
		db.execSQL(sql.toString());
	}

	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
		Log.w(WordSQLiteHelper.class.getName(),"Upgrading database from version " + oldVersion + " to "
	            + newVersion + ", which will destroy all old data");
		db.execSQL("DROP TABLE IF EXISTS "+TABLE_WORDS);
		onCreate(db);
	}
}

在做web应用时，为了代码层次能够更加清晰，通常将其划分为3层结构(entity、dao和service)
dao的创建使得Service的编写变得更加容易，android应用可基于SQLiteDatabase对象构建出自己的Dao实例

public class WordDao implements IBaseDao<Word>{
	private SQLiteDatabase database;
	private WordSQLiteHelper dbHelper;
	private String[] allColumns={WordSQLiteHelper.COLUMN_ID,WordSQLiteHelper.COLUMN_ENGLISH,
			WordSQLiteHelper.COLUMN_CHINESE,WordSQLiteHelper.COLUMN_PHRASE,WordSQLiteHelper.COLUMN_TRANSLATION,WordSQLiteHelper.COLUMN_LEVEL};
	public WordDao(Context context){
		dbHelper=new WordSQLiteHelper(context);
	}
	public void open(){
		database=dbHelper.getWritableDatabase();
	}
	public void close(){
		dbHelper.close();
	}
	@Override
	public void create(Word word){
		ContentValues values = new ContentValues();
		values.put(WordSQLiteHelper.COLUMN_ENGLISH,word.getEnglish());
		values.put(WordSQLiteHelper.COLUMN_CHINESE,word.getChinese());
		values.put(WordSQLiteHelper.COLUMN_PHRASE,word.getPhrase());
		values.put(WordSQLiteHelper.COLUMN_TRANSLATION,word.getTranslation());
		values.put(WordSQLiteHelper.COLUMN_LEVEL,word.getLevel());
		long insertId=database.insert(WordSQLiteHelper.TABLE_WORDS,null,values);
		word.setId(insertId);
	}
	@Override
	public void update(Word word){
		ContentValues values = new ContentValues();
		values.put(WordSQLiteHelper.COLUMN_ENGLISH,word.getEnglish());
		values.put(WordSQLiteHelper.COLUMN_CHINESE,word.getChinese());
		values.put(WordSQLiteHelper.COLUMN_PHRASE,word.getPhrase());
		values.put(WordSQLiteHelper.COLUMN_TRANSLATION,word.getTranslation());
		values.put(WordSQLiteHelper.COLUMN_LEVEL,word.getLevel());
		database.update(WordSQLiteHelper.TABLE_WORDS,values,WordSQLiteHelper.COLUMN_ID+"="+word.getId(),null);
	}
	@Override
	public void delete(Word word){
		long id=word.getId();
		database.delete(WordSQLiteHelper.TABLE_WORDS,WordSQLiteHelper.COLUMN_ID+"="+id,null);
	}
	@Override
	public List<Word> getAll(){
		List<Word> words=new ArrayList<Word>();
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		//Cursor cursor=database.rawQuery("select * from "+WordSQLiteHelper.TABLE_WORDS,new String[]{});
		cursor.moveToFirst();
		while(!cursor.isAfterLast()){
			words.add(encapsulate(cursor));
			cursor.moveToNext();
		}
		cursor.close();
		return words;
	}
	@Override
	public List<Word> findOnePage(int offset,int num){
		List<Word> words=new ArrayList<Word>();
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		cursor.moveToFirst();
		cursor.move(offset);
		int i=0;
		while(!cursor.isAfterLast()){
			if(i==num){
				break;
			}
			words.add(encapsulate(cursor));
			cursor.moveToNext();
			i++;
		}
		cursor.close();
		return words;
	}
	@Override
	public int getCount(){
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		return cursor.getCount();
	}
	private Word encapsulate(Cursor cursor){
		Word word=new Word();
		word.setId(cursor.getLong(0));
		word.setEnglish(cursor.getString(1));
		word.setChinese(cursor.getString(2));
		word.setPhrase(cursor.getString(3));
		word.setTranslation(cursor.getString(4));
		word.setLevel(cursor.getFloat(5));
		return word;
	}
}

不同于Hibenrate的处理方式，这里的dao需要手动开启和关闭session(通过执行open和close方法)，并且每个dao实例只封装一个session对象。

在web应用中，dao实例一般通过spring容器来管理的，引入容器确保了dao在系统中只存在一个实例，由于每个dao都封装了一个session，单实例的dao也避免了session资源的浪费。
我们可效仿spring的管理方式，编写自己的dao容器，同时，在容器中声明clear方法，当系统APP退出时，执行该方法对容器进行销毁，销毁容器的同时，关闭dao对应的session。

public class DaoContext {
	private Map<String,IBaseDao<?>> daoMap;
	private static DaoContext instance=new DaoContext();
	private DaoContext(){
		daoMap=new HashMap<String,IBaseDao<?>>();
	}
	public static DaoContext getInstance(){
		return instance;
	}
	public void addDao(IBaseDao<?> dao){
		String daoName=dao.getClass().getSimpleName();
		removeDao(daoName);
		daoMap.put(dao.getClass().getSimpleName(),dao);
		dao.open();
	}
	public void removeDao(String daoName){
		if(daoMap.containsKey(daoName)){
			daoMap.remove(daoName);
			IBaseDao<?> dao=getDao(daoName);
			dao.close();
		}
	}
	public IBaseDao<?> getDao(String daoName){
		return daoMap.get(daoName);
	}
	public void clear(){
		for(Entry<String, IBaseDao<?>> entry:daoMap.entrySet()){
			IBaseDao<?> dao=entry.getValue();
			dao.close();
		}
		daoMap.clear();
	}
}